System for extracting lithium carbonate from saline lake water

ABSTRACT

The present invention discloses a method for quickly extracting lithium carbonate from saline lake water and a system for the same. The method comprises: first quick-freezing the saline lake water to obtain lithium-rich brine, then evaporating under reduced pressure to enable lithium carbonate to be rapidly precipitated out. The method has advantages of short process flow and less labor consumption, thereby enabling continuous automatic operation, high energy utilization and environment-friendly. Further, the crystallization rate is several times faster than that of the salt-pan process and the grade of lithium carbonate salt mine obtained can reach 95% or more, therefore the method of the present invention is particularly suitable for industrial production in the remote saline lake region. The system comprises a reduced-pressure evaporation crystallizer, a vacuum-pumping apparatus, a brine preheating apparatus and a brine cooling apparatus, which concentrates the brine by quick-evaporation of the water, promotes lithium carbonate to form non-uniform nucleus, and improves the crystallization efficiency.

FIELD OF THE INVENTION

The present invention relates to a method and a system for extractinglithium carbonate from a saline lake, specifically it relates to amethod and a system for quickly extracting lithium carbonate from salinelake water.

BACKGROUND OF THE INVENTION

The saline lake contains many mineral salts and is an important sourceof mineral salts. However, large-scale exploitation of saline lake isdifficult because the content of mineral salts in the saline lake isgenerally low. The main mining technologies of recent years areevaporation of saline lake water in the sun to obtain salt, depositmining and the like, in which evaporation of saline lake water in thesun is the most environment-friendly exploitation method. Furtherdevelopment is seriously restricted because of its low efficiency.Therefore, a method which can improve the efficiency of the evaporationcrystallization will have important strategic value.

Lithium is a metallic element having the lowest atomic weight. Itpossesses special properties such as the most negative potential, thegreatest electrochemical equivalent, high specific heat, highconductivity, and strong chemical reactivity etc. It is an idealmetallic material for manufacturing disposable batteries, rechargeablepower batteries, and structural material for aerospace etc. Thus, it ishonored as the energy metal in the 21^(st) century.

In nature, lithium exists mainly in the form of pegmatite such asspodumene, lepidolite etc., and in the form of lithium ions in thesaline lake brine, underground brine and seawater. The saline lakelithium resource reserves account for more than 69° A of the worldindustrial lithium reserves, while the saline lake lithium resourcereserves of China account for 85° A of the industrial lithium reserves.Extracting lithium from saline lake brine possesses features such assimple technology, low energy consumption, and low cost. It hasgradually substituted the production of lithium from lithium ore.Presently, there are many ways to extract lithium from saline lake, forexample, ion exchange, adsorption, carbonization, precipitation, andextraction. However, these methods are immature, costly, or cannot meetthe environmental requirements, thus, making it difficult to realizeindustrial production.

The current industrial production of lithium salt from saline lake inthe country mostly employs salt-pan process, including enriching thelithium by brine evaporation in the drying bed in the winter, thenpouring the lithium-rich brine into a solar pond for heat accumulationand retention until the temperature reaches between 30° C. and 50° C.More lithium carbonates reach oversaturation and precipitate as thesolubility of lithium carbonate in brine is decreased with increasingtemperature. Then, upon further chemical processing, the industrialgrade of lithium carbonate is obtained. Although this process takesadvantage of the superiority of plateau solar energy and cold energy,its production cycle is too long (the brine evaporation stage takes 4 to6 months, the crystallization stage takes 2 to 4 months), leading toonly one harvest a year, and the production efficiency is very low,leading to little benefit. Moreover, the construction of the solar pondrequires lots of costs and materials, the geomembrane for heatpreservation laying at the bottom of the pond is costly despite havingpoor leakage resistance. Therefore, a significant loss of brine resultsfrom the leakage problem of the geomembrane and it adversely affects theproductivity and economic benefits.

The constituent of brine is very complex, having low content of lithiumas compared to that of sodium and potassium. Therefore, it is necessaryto concentrate the lithium before extraction so as to improve the gradeof the lithium salt. The solubility of lithium carbonate in waterdecreases with temperature increase. This is referred to as an inversesolubility property. Experiment indicates that the solubility of lithiumcarbonate in brine has a similar property, while sodium salt andpotassium salt have a positive solubility property. Therefore, thefreezing method can be used to precipitate a large amount of sodium saltand potassium salt at low temperature, causing the preliminaryenrichment of lithium in brine, then directly heat the lithium-richbrine to achieve the purpose of extracting high-grade lithium carbonate.The relational experiments show that sodium salt and potassium salt mayprecipitate out more quickly by quick-freezing than by general-freezing.Therefore, a higher enrichment efficiency of lithium can be achieved byusing quick-freezing. If the concentration of lithium carbonate in brinehas not reached saturation concentration at a certain temperature, theneven if the temperature of the brine is higher, the lithium carbonatewill not crystallize. As a result, it will have to be kept warm longerto allow the lithium salt reach saturation concentration and precipitateout. In order to shorten the holding time and accelerate the enrichmentof lithium in brine, a reduced-pressure evaporation can be used.

Reduced-pressure evaporation is an efficient way for increasingevaporation rate. With regard to a water-containing solution or avolatile solvent-containing solution in a sealed container, theequilibrium between the liquid phase and the gas phase is balanced. Whenthe system pressure decreases, i.e., after the gas phase on the surfaceof the liquid is removed, the equilibrium between the gas phase and theliquid phase is broken. Then, the water or volatile solute in the liquidphase tends to evaporate to supplement the gas phase so as to maintainthe gas-liquid equilibrium. Thereby the boiling point of the solutiondecreases. The liquid phase will keep boiling and evaporating if theevaporated gas is continuously removed, and the solute will getcontinuously concentrated. When the solute concentration reaches thesaturation concentration of this temperature, the solute willcrystallize.

Many devices and methods have been reported for extracting Li₂CO₃ fromsaline lake brine both at home and abroad, for example, “AdsorptionTechniques and Progress on the Extraction of Lithium from Salt LakeBrines” (Journal of Salt and Chemical Industry, vol. 36(3), 2007),“Preparation of Lithium Carbonate from Brines by Solvent Extraction”(Journal of Salt lake Research, vol. 14(2), 2006), “Progresses on theProcess and Technique of Lithium Recovery from Salt Lake Brines Aroundthe World” (World Sci-Tech R & D, vol. 28(5), 2006), “The Investigationon Lithium in the Bitterns of Enrichment by Solvent Flotation” (Journalof Salt and Chemical Industry, vol. 40(1), 2011) and the like. Thesedevices and methods have the advantages that the adsorption method takesadvantage of an ionic sieve type oxide by using its good selectiveadsorption property towards lithium characterized by a selectivecoefficient that can reach up to 10⁴ to 10⁵. Thus, it can be effectivelyused to extract lithium from saline lake brine, and its mechanicalstrength and chemical stability are excellent. Solvent extraction issuitable for saline lake brine having higher content of magnesiumchloride, characterized by its simple technology, high product purity,and lower energy consumption. The carbonate precipitation method is asimple process, with high reliability. It is suitable for saline lakebrine having a low ratio of magnesium to lithium. The disadvantages ofthe methods mentioned above are that: there are great differencesbetween the practical and the theoretical adsorptive capacity of theionic sieve type adsorbent; the poor permeability and difficulty togranulate restricts the industrial application of this method; thedevice is susceptible to corrosion; the loss of extracting agent isgreat in solvent extraction method; and low extraction efficiency in thegeneral precipitation method. None of the methods mentioned above canachieve higher productivity while protecting the environment.

There still exists a need in the art to get a crystallization method anda device for effectively extracting lithium salt from saline lake brinewhich are also environment-friendly.

SUMMARY OF THE INVENTION

In view of the disadvantages of the salt pan technology in extractinglithium, such as high cost, not environment-friendly, long productioncycle, the present invention aims to provide a method and a system forquickly extracting lithium carbonate from saline lake water.

The technical solution adopted by the present invention is that:

A method for quickly extracting lithium carbonate from saline lakewater, comprising the following steps:

1) introducing the lithium-rich brine into a reduced-pressureevaporation crystallizer to enable lithium carbonate to reachoversaturation and precipitate from the lithium-rich brine byreduced-pressure evaporation;

2) discharging the residual liquid, collecting the precipitate in thecrystallizer and drying the precipitate to obtain lithium carbonatecrystals.

As a further improvement of the present invention, during thereduced-pressure evaporation, the pressure of the reduced-pressureevaporation crystallizer is maintained between 25 kPa and 45 kPa.

As a further improvement of the present invention, the temperature forreduced-pressure evaporation ranges from 60° C. to 80° C.

As a further improvement of the present invention, the lithium-richbrine is preheated before being introduced into a reduced-pressureevaporation crystallizer.

As a further improvement of the present invention, the preparation ofthe lithium-rich brine comprises: cooling saline lake brine to −40°C.˜−20° C. to precipitate the sodium and potassium salt out of thebrine; and separating the solids from the liquid to obtain thelithium-rich brine.

A system for quickly extracting lithium carbonate from saline lakewater, comprising a reduced-pressure evaporation crystallizer, avacuum-pumping apparatus, a brine preheating apparatus and a brinecooling apparatus. The reduced-pressure evaporation crystallizer has anouter wall, an inner container and an upper cover, a heater is equippedbetween the outer wall and the inner container, the top of the uppercover is equipped with a steam outlet and a brine inlet, the bottom ofthe upper cover is equipped with several detachable fins, and the steamoutlet is connected with the vacuum-pumping apparatus. Thevacuum-pumping apparatus is connected with the brine preheatingapparatus, and the brine preheating apparatus is connected with thebrine cooling apparatus via pipes.

As a further improvement of the present invention, the fins areconfigured to have cannelures.

As a further improvement of the present invention, the heater isequipped around outside of the inner container.

As a further improvement of the present invention, a heat exchanger usedfor pre-cooling the saline lake brine is equipped between the brinepreheating apparatus and the brine cooling apparatus.

As a further improvement of the present invention, the heater is anelectric heater.

The beneficial effects of the present invention are as follows:

1) by use of the abundant solar power in the plateau area to generateelectricity for quick-freezing the brine, and subsequently heat thebrine, the energy is clean and environment-friendly;

2) making full use of the inverse solubility characteristics of lithiumcarbonate and the positive solubility property of the sodium salt andpotassium salt. Creatively using quick-cooling first followed byquick-heating technology by precipitating out the sodium and potassiumsalt, in the brine, quickly at a very low temperature, consequentlyenriching the concentration of lithium ion in the brine, and achievingthe goals of separating impurities and enriching lithium;

3) during the reduced-pressure evaporation crystallization, controllingthe temperature and pressure constant not only to enable the brine tocontinuously boil, evaporate and concentrate, but also to enable lithiumcarbonate to reach saturation more easily and precipitate out.Furthermore, the added fin devices increase the non-uniformly nucleationarea of lithium carbonate. The residual lithium carbonate in thecannelures of fins may work as cores during next nucleation process,which contributes to quick nucleation and crystallizing of lithiumcarbonate;

4) the grade of lithium carbonate obtained by the method of the presentinvention is especially high and can reach 95% or even more; theprecipitation rate of lithium carbonate can reach 99% which saves alarge amount of energy. The discharged liquid contains almost no lithiumwhich ensures the high usage rate of the brine;

5) as the method of the present invention is a purely physical processwithout using any chemicals, it is environment friendly. Continuousproduction, mechanization and automation during the whole-course can berealized by the method of the present invention, therefore the laborcost is reduced.

The device of the present invention can significantly promote waterevaporation and make full use of the heat, therefore it has advantagesof good acceleration effect and high crystallization efficiency. Inaddition, the acceleration process is a purely physical process, whichis environment-friendly and green. Most of lithium carbonate obtained bycrystallization adhere to the fins and is easily collected. Fresh watercan be produced during the process of reduced-pressure evaporation,which is particularly suitable for remote saline lake mining area.

The fins are configured to have cannelures which would further increasethe non-uniform nucleation area of Li₂CO₃, promote the nucleation andcrystallization of Li₂CO₃, and increase the crystallization efficiencyof Li₂CO₃.

After the vacuum-pumping apparatus is connected, the vapor pressure ofthe brine surface can be further decreased which accelerates thecrystallization. The recycled steam enters into the brine preheatingapparatus, efficiently making use of the energy.

The heater is equipped around the outside of the inner container whichmakes the saline lake brine self-circulate and develop a countercurrentflow after being heated, causing more uniform heating, while avoidinglithium carbonate fouling on the wall of the inner container toadversely affects heating efficiency.

A heat exchanger used for pre-cooling the saline lake brine is equippedbetween the brine preheating apparatus and the brine cooling apparatus,in which the saline lake brine is effectively pre-cooled, and thelithium-rich brine is heated up at the same time. Thereby, the energyconsumption required for cooling and preheating can be reduced.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural schematic diagram of the device of the presentinvention;

FIG. 2 is a structural schematic diagram of the fins of the device ofthe present invention.

DETAILED DESCRIPTION

The present invention will be further illustrated below with referenceto the accompanying drawings and embodiments.

Referring to FIGS. 1˜2, a system for quickly extracting lithiumcarbonate from saline lake water, comprising a reduced-pressureevaporation crystallizer, a vacuum-pumping apparatus 5, a brinepreheating apparatus 6 and a brine cooling apparatus 7. Thereduced-pressure evaporation crystallizer has an outer wall 1, an innercontainer 2 and an upper cover 3. Heater 4 is equipped between the outerwall 1 and the inner container 2. The top of the upper cover 3 isequipped with a steam outlet 31 and a brine inlet 32, the bottom ofwhich is equipped with several detachable fins 33. The steam outlet 31is connected with the vacuum-pumping apparatus 5 which connected withthe brine preheating apparatus 6, and the brine preheating apparatus 6is connected with the brine cooling apparatus 7 via pipes.

As a further improvement of the present invention, the fins 33 areconfigured to have cannelures.

As a further improvement of the present invention, the heater 4 isequipped around outside the inner container 2. In particular, the heateris an electric heater.

As a further improvement of the present invention, a heat exchanger usedfor pre-cooling saline lake brine is equipped between the brinepreheating apparatus and the brine cooling apparatus.

The electric energy used in the system of the present invention comesfrom solar power station. In the remote plateau area, the abundant solarenergy resource can be used effectively, which is green andenvironment-friendly.

The fins are configured to have cannelures which further increase thenon-uniformly nucleation area of Li₂CO₃, promote the nucleation andcrystallization of Li₂CO₃, thereby increase the crystallizationefficiency of Li₂CO₃.

After the vacuum-pumping apparatus is connected, the vapor pressure ofthe brine surface can be further decreased which accelerates thecrystallization. The recycled steam enters into the brine preheatingapparatus, making more efficient use of the energy.

The heater is equipped around the outside of the inner container whichmakes the saline lake brine self-circulate and develop a countercurrentflow after being heated, causing more uniform heating, while lithiumcarbonate fouling on the wall of the inner container, which adverselyaffects heating efficiency, can be avoided.

During operation, the saline lake brine is cooled in brine coolingapparatus 7. Preferably, filtrated lithium-rich brine is pre-cooled asit passes through the heat exchanger, which increases the temperature ofthe lithium-rich brine at the same time. Subsequently, the lithium-richbrine enters into the brine preheating apparatus 6 to be pre-heated, andthen enters into the inner container 2 of the reduced-pressureevaporation crystallizer via the brine inlet 32. Then vacuum-pumpingapparatus 5 is switched on to reduce the vapor pressure of the innercontainer 2, so as to accelerate evaporation and crystallization.Pumped-out steam enters into the brine preheating apparatus 6, and iscollected by the via pipe 8.

Embodiment 1

1) The brine of North lake of Zhabuye saline lake taken in summer (theconcentration of lithium ion is 0.78 g/L, and the initial temperature ofthe brine is 5° C. is cooled to −40° C. by industrial freezing system.After the precipitation of sodium chloride, potassium chloride, andaphthitalite, a solid-liquid separation is carried out to obtain thelithium-rich brine;

2) The obtained lithium-rich brine is transferred into thereduced-pressure evaporation crystallizers after being pre-heated by thepreheating apparatus, wherein each crystallizer contains 30 L brine;

3) The brine is quickly heated to 60° C., evaporated at areduced-pressure for 5 min, and the pressure of the crystallizer isbalanced at 26 kPa. After 2 hours, a lithium-rich salt mixture having athickness of about 2 mm has been deposited on the bottom of thecrystallizer, and the residual brine is discharged.

The grade of obtained lithium carbonate in the mixture is 95.2%, and theprecipitation rate of the lithium salt is 99.0%.

The operation is run continuously for seven days following the aboveprocedure. The grade of lithium carbonate obtained from the saltcrystallized in the reduced-pressure evaporation crystallizer is 95.2%and the mixture is of 3.55 tons. The time required for producing lithiumcarbonate is 1.97 days per ton on average.

Embodiment 2

1) The brine of North lake of Zhabuye saline lake taken in summer (theconcentration of lithium ion is 0.54 g/L, and the initial temperature ofthe brine is 8° C.) is cooled to −30° C. using an industrial freezingsystem. After precipitation of sodium chloride, potassium chloride, andaphthitalite, a solid-liquid separation is carried out to obtain thelithium-rich brine;

2) The obtained lithium-rich brine is transferred into thereduced-pressure evaporation crystallizers after being pre-heated by thepreheating apparatus, wherein each crystallizer contains 30 L brine;

3) The brine is quickly heated to 70° C., evaporated at areduced-pressure for 5 min, and the pressure of the crystallizer isbalanced at 36 kPa. After 2 hours, a lithium-rich salt mixture having athickness of about 2 mm has been deposited on the bottom of thecrystallizer, and the residual brine is discharged.

The grade of obtained lithium carbonate in the mixture is 95.8%, and theprecipitation rate of the lithium salt is 99.6%.

The operation is run continuously for seven days following the aboveprocedure. The grade of lithium carbonate obtained from the saltprecipitated in the reduced-pressure evaporation crystallizer is 95.8%and the mixture is of 3.80 tons. The time required for producing lithiumcarbonate is 1.84 days per ton on average.

Embodiment 3

1) The brine of salt-span of Zhabuye saline lake which has been dried inthe sun for 10 days (the concentration of lithium ion is 1.2 g/L, andthe initial temperature of the brine is 5° C.) is cooled to −20° C. byindustrial freezing system. After precipitation of sodium chloride,potassium chloride, and aphthitalite, a solid-liquid separation iscarried out to obtain the lithium-rich brine;

2) The obtained lithium-rich brine is transferred into thereduced-pressure evaporation crystallizers after being pre-heated by thepreheating apparatus, wherein each crystallizer contains 30 L brine;

3) The brine is quickly heated to 75° C., evaporated at areduced-pressure for 5 min, and the pressure of the crystallizer isbalanced at 40 kPa. After 2 hours, a lithium-rich salt mixture having athickness of about 4 mm has been deposited on the bottom of thecrystallizer, and the residual brine is discharged.

The grade of obtained lithium carbonate in the mixture is 96.6%, and theprecipitation rate of the lithium salt is 99.0%.

The operation is run continuously for seven days following the aboveprocedure. The grade of lithium carbonate obtained from the saltprecipitated in the reduced-pressure evaporation crystallizer is 96.6%and the mixture is of 4.01 tons. The time required for producing lithiumcarbonate is 1.75 days per ton on average.

Embodiment 4

1) The brine of salt-span of Zhabuye saline lake which has been dried inthe sun for 60 days (the concentration of lithium ion is 2.1 g/L, andthe initial temperature of the brine is 12° C.) is cooled to −20° C. byindustrial freezing system. After precipitation of sodium chloride,potassium chloride, and aphthitalite, a solid-liquid separation iscarried out to obtain the lithium-rich brine;

2) The obtained lithium-rich brine is transferred into thereduced-pressure evaporation crystallizers after being pre-heated by thepreheating apparatus, wherein each crystallizer contains 30 L brine;

3) The brine is quickly heated to 80° C., evaporated at areduced-pressure for 5 min, and the pressure of the crystallizer isbalanced at 45 kPa. After 2 hours, a lithium-rich salt mixture having athickness of about 5 mm has been deposited on the bottom of thecrystallizer, then the residual brine is discharged.

The grade of obtained lithium carbonate in the mixture is 97.3%, and theprecipitation rate of the lithium salt is 99.8%.

The operation is run continuously for seven days following the aboveprocedure. The grade of lithium carbonate obtained from the saltprecipitated in the reduced-pressure evaporation crystallizer is 97.3%and the mixture is of 4.28 tons. The time required for producing lithiumcarbonate is 1.63 days per ton on average.

As can be seen from the data of the above embodiments, lithium carbonatecan be quickly and effectively extracted from the saline lake brine. Thegrade of lithium carbonate obtained by the method of the presentinvention is especially high, even can reach 95% or more, and the totalrecovery can reach 99%.

The device of the present invention can significantly promote moistureevaporation and make full use of the heat, therefore has advantages ofgood acceleration effect and high crystallization efficiency. Theacceleration process is a purely physical process, which isenvironment-friendly and green. Most of lithium carbonate obtained bycrystallization adhere to the fins and are easy to be collected. Whenthe crystallization process of lithium carbonate is accelerated, thefresh water can also be produced, which is particularly suitable forremote saline lake mining area.

The invention claimed is:
 1. A system for extracting lithium carbonatefrom saline lake water, the system comprising: a reduced-pressureevaporation crystallizer; a vacuum-pumping apparatus; a brine preheatingapparatus; and a brine cooling apparatus, wherein the reduced-pressureevaporation crystallizer comprises an outer wall, an inner container andan upper cover, a heater equipped between the outer wall and the innercontainer, wherein the upper cover comprises a top and a bottom, whereinthe top of the upper cover is equipped with a steam outlet and a brineinlet and the bottom of the upper cover is equipped with severaldetachable fins, wherein the steam outlet is connected with thevacuum-pumping apparatus via pipes, wherein the vacuum-pumping apparatusis connected with the brine preheating apparatus via pipes, and whereinthe brine preheating apparatus is connected with the brine coolingapparatus via pipes.
 2. The system according to claim 1, wherein thefins are configured to have cannelures.
 3. The system according to claim1, wherein the heater is equipped outside of the inner container.
 4. Thesystem according to claim 1, wherein a heat exchanger used forpre-cooling the saline lake water is equipped between the brinepreheating apparatus and the brine cooling apparatus.
 5. The systemaccording to claim 1, wherein the heater is an electric heater.
 6. Thesystem according to claim 2, wherein the heater is equipped outside ofthe inner container.
 7. The system according to claim 2, wherein a heatexchanger used for pre-cooling the saline lake water is equipped betweenthe brine preheating apparatus and the brine cooling apparatus.
 8. Thesystem according to claim 2, wherein the heater is an electric heater.